1. Field of the Invention
The present invention relates to a stage for placing thereon a substrate, such as a semiconductor substrate, a glass substrate for a liquid crystal display device, a glass substrate for a photomask, a glass substrate for a plasma display, a substrate for an optical disk and the like, and to a heat treatment apparatus using the same.
2. Description of the Background Art
A substrate processing apparatus (for example, a heat treatment apparatus, a cleaning processing apparatus, a development processing apparatus, and the like) for performing various processes on a substrate requires a stage for placing a substrate to be processed thereon. In particular, stages for use in a heat treatment apparatus are often made of a metal having a high thermal conductivity, e.g. aluminum. The stages made of aluminum, however, have disadvantages to be described below.
One of the disadvantages is that such an aluminum stage has poor wear resistance. Because aluminum is a relatively soft material, the continued use of the stage with aluminum exposed at its holding surface causes the holding surface to be easily worn away after the repeated placement of substrates on the holding surface. As a result, the stage made of aluminum is poor in durability. There arises another problem in that the stage becomes a source of contaminants due to the wear of the stage itself.
Another disadvantage lies in the noticeable occurrence of separation electrification. The “separation electrification” is a phenomenon such that a negative charge (or a positive charge) is generated on a substrate made of an insulating material such as glass when the substrate is removed or separated from a stage made of metal which is a conductive material. The negative charge (or the positive charge) generated on the substrate causes a damage to a device formed on the surface of the substrate, the adsorption of contaminants on the surface of the substrate, and the like. For this reason, the prevention of the separation electrification is a significant challenge in the stage made of aluminum. It should be noted that whether the substrate is charged negatively or positively is determined by the combination of the material of the substrate and the material of the stage, that is, triboelectric series.
To overcome the above-mentioned disadvantages of the stage made of aluminum, a variety of techniques have been devised.
For example, there has been devised a technique for reducing the contact area between the substrate and the stage step by step to thereby suppress static electricity produced during the separation of the substrate from the stage. This technique employs a configuration such that the holding surface of the stage is divided into parts which in turn are sequentially separated from the substrate, to thereby achieve the step-by-step separation of the substrate (as disclosed in, for example, Japanese Patent Application Laid-Open No. 11-251257 (1999)).
As another example, there has been devised a technique for forming a film of alumina (Al2O3) which is an insulator on the surface of the stage made of aluminum to improve the wear resistance of the stage and to prevent the separation electrification. An example of this technique includes spraying aluminum oxide particles substantially at subsonic speed toward a position for coating to deposit the particles in the form of fine crystallites joined to each other in that position, thereby forming a thin film of alumina having a polycrystalline structure (as disclosed in, for example, Japanese Patent Application Laid-Open No. 2002-190512).
The conventionally devised techniques produce some degree of effect in improving the wear resistance of the substrate stage and in reducing the separation electrification. None of these techniques, however, have achieved the sufficient effect.
For example, the technique disclosed in Japanese Patent Application Laid-Open No. 11-251257 (1999) shows that the step-by-step separation of the substrate from the stage lowers the separation electrification relative to the separation of the entire substrate at a time from the stage. This technique, however, cannot completely prevent the separation electrification.
The technique disclosed in Japanese Patent Application Laid-Open No. 2002-190512 shows an attempt to reduce the influence of a grain boundary layer. However, the influence of the grain boundary layer cannot be completely eliminated because of the use of the polycrystalline structure. When the alumina film having the polycrystalline structure is employed, the repeated use of the stage creates voids at the boundary (grain boundary) between individual crystal grains to cause a crack to occur. In particular, when the alumina film undergoes a thermal shock, such a crack occurs noticeably to finally result in the removal of the alumina film from the stage. Also, the voids provide electrical connection between the substrate and an aluminum region of the stage to spoil the effect of preventing the separation electrification.